as far as i know, its not an option -- thats the gig.
but why would you want to switch to just one racer? its really not that big of a difference. its just that now, instead of one character driving and shooting shells, it'll be one guy driving and the other guy shooting.
control wise, for you, its the same thing -- but animation wise, on screen, it'll look so much cooler.
instead of a shell just "following" you or launching, amazingly, from the underneath of your cart, you'll actually see someone chucking it or dropping it or laughing when it connects with someone else.
its an unnecessary change, since its not really affecting the gameplay for the user, but definitely a welcome one to add the extras!!
